The liquid-crystal display apparatus is a flat-panel display apparatus having superior features such as high definition, thin type, lightweight, and low power consumption, and is widely used for a flat panel television, PC monitor, digital signage, and others. In the flat display apparatus of the active matrix type, a data signal is written in a pixel electrode in individual pixels included in the liquid-crystal panel (which is also simply referred to as the panel hereinafter) from data-signal bus line through thin-film transistor (TFT) in selection periods for each pixel.
In the selection periods for the respective pixels, scan signal as selection signal is applied from scan-signal line to a gate of a TFT of each pixel. In this case, it is known that when the driving voltage for the gate rises, a feed-through voltage (so-called pull-in voltage) occurs due to the influence of the parasitic capacitance between the gate and the drain of the TFT, and the voltage of the pixel electrode lowers below a voltage of the data signal (See Non-patent Document 1).
For example, when the parasitic capacitance between the gate and the drain of the TFT and the parasitic capacitance between the source and the drain of the TFT are respectively denoted by Cgd and Csd, and the capacitance of each pixel (which corresponds to the sum of the liquid-crystal capacitance, the auxiliary capacitance connected in parallel with the liquid-crystal capacitance, and the parasitic capacitances Cgd and Csd) is denoted by Cpx, the above pull-in voltage ΔVd is expressed by the following expression (1):ΔVd=(Cgd/Cpx)×(VgH−VgL),  (1)
where VgH denotes the voltage when the scan signal is at the high level, and
VgL denotes the voltage when the scan signal is at the low level.
Since the actual scan-signal line is regarded as distributed constant line having a reactance component and a resistance component, the waveforms of the scan signal is more deformed with distance from a driving end. Therefore, the magnitude of the pull-in voltage expressed by the expression (1) varies with the position on the panel in the direction along the scan-signal line.
FIG. 1 is an explanation drawing for explaining voltage of a scan signal and voltage of a pixel electrode in a conventional liquid-crystal display apparatus. For a panel edge part and a panel center part in the direction along the scan-signal lines, the waveform of the voltage of a scan-signal and the waveform of the voltage of a pixel electrode are respectively shown on the upper and lower sides of FIG. 1. The abscissa in FIG. 1 indicates time. The scan signal is driven from both of the right and left edges. In FIG. 1, Vs+ and Vs− respectively indicate the signal levels of the positive data signal and negative data signal.
At the edge part of the panel, i.e., at the driving end for the scan signal, the scan signal rapidly fall, and the voltage of the pixel electrode is lowered ΔVd0 below the voltage of the data signal (Vs+ and Vs−) due to the pull-in voltage corresponding to the amplitudes of the fall of the scan signal. The amount ΔVd0 corresponds to the value expressed by the expression (1). In FIG. 1, the voltages of the pixel electrode in the case where the positive data signal is written in the pixel electrode and in the case where the negative data signal is written in the pixel electrode are shown in an overlapped manner.
On the other hand, at the central part of the panel, because of occurrence of deformation in rise and fall of the scan signal, each TFT is turned on and a data signal is written in the pixel electrode when the voltage of the scan signal exceeds a voltage which is higher than the voltage of the data signal (Vs+ or Vs−) by the threshold level of the TFT. Thereafter, the TFT is turned off when the voltage of the scan signal falls below the voltage higher than the voltage of the data signal by the threshold level of the TFT. In FIG. 1, for simplicity purpose, the threshold level of the TFT is assumed to be 0 V. As shown in the drawing, at the central part of the panel, when a positive (or negative) data signal is written, it takes a time Tf1 (or Tf2) until the TFT is turned off after the scan signal starts to rise.
Since the TFT is slowly turned from ON to OFF during the time Tf1 (or Tf2), transfer of electric charge (so-called recharging) occurs between the signal line for the data signal and the pixel electrode, and a pull-in voltage ΔVd1 (or ΔVd2), smaller than ΔVd0, occurs. The magnitude of ΔVd1 (or ΔVd2) decreases with increase in the time Tf1 (or Tf2), in which the recharging occurs. That is, the closer to the central part of the panel than the edge part the pixel is, the smaller the magnitude of the pull-in voltage taking into account the recharging is, and the amount of lowering of the voltage of the pixel electrode also becomes smaller. In addition, since the pull-in voltage taking into account the recharging is smaller in the case where the negative data signal is written than in the case where the positive data signal is written, the closer to the central part of the panel than the edge part the pixel is, the smaller the amplitude of the effective voltage of the pixel electrode is, and the brightness of the pixel becomes lower.
FIG. 2 is an explanation drawing for explaining deviation of counter voltage and luminance non-uniformity in a conventional liquid-crystal display apparatus. Distributions of the voltages of pixel electrodes and luminance non-uniformity are respectively shown in the upper part and lower part of FIG. 2. In the upper part of FIG. 2, the distributions in a case where positive data signal is written in the pixel electrode and in a case where negative data signal is written in the pixel electrode are shown together by solid lines. The abscissa in FIG. 2 indicates the distance from the left edge of the panel. In FIG. 2, Vcom represented by the dash-dot line indicates the counter voltage, i.e., the voltage level of the counter electrode. The scan signal is assumed to be driven from both of the right and left edges of the panel.
Because of the characteristics of the pull-in voltage taking into account the above-mentioned recharging, the voltage distribution of pixel signals draws an upward convex curve which is minimized at both ends of the panel and is maximized in the central part of the panel. Generally, the counter voltage is adjusted to an optimum counter voltage in the middle between the positive data signal and the negative data signal which are written in pixel electrode. In the case where the voltages of the pixel signals have a distribution characteristic as shown in the upper part of FIG. 2, the optimum counter voltage should vary drawing an upward convex curve as shown by the dashed line. However, since the conventional counter voltage is set to a constant voltage over the entire surface on the panel, if the counter voltage is set focusing the central part of the panel, a counter voltage which is greatly biased to the positive side compared with the optimum counter voltage is applied in the edge parts of the panel. In addition, since the brightness of the pixels has a distribution as described above, luminance non-uniformity in which the screen is displayed such that the edge parts of the panel are brighter than the central part occurs as shown in the lower part of FIG. 2.
As described above, the magnitude of the pull-in voltage ΔVd and the amplitude of the effective voltage show the distribution characteristic of decreasing depending on the distance from the driving end for the scan signal, and are uneven in the panel surface. Therefore, inexpediency exists, which may be, for example, occurrence of the luminance non-uniformity in respective pixels or occurrence of a flicker depending on the frame rate. Further, since the optimum counter voltage for each pixel varies with the distance from the driving end for the scan signal, uniform AC driving of the liquid crystal within the panel surface becomes impossible, thereby, burn-in due to application of DC components to the liquid crystal is caused.
In view of above, in Patent Document 1, there is proposed a liquid-crystal panel in which a gate driver (driver circuit) is connected to one end of a scan-signal line, and a discharge circuit is connected to the other end of the scan-signal line, thereby, the other end is opened when a scan signal applied from the one end of the scan-signal line is in an ON level, and when the scan signal is in an OFF level, a control voltage for OFF is also applied from the other end. In addition, in Patent Document 2, there is proposed a liquid-crystal panel in which waveforms of the falling edges of the scan signal at the driving end for the scan signal and at the terminating end are approximately equalized without being affected by the propagation delay characteristic of the scan-signal line, by making the falling edge of the scan signal have a waveform of an approximately straight inclined line. Further, Patent Document 3 proposes a driving circuit for a liquid-crystal display apparatus. The driving circuit adds, to a data signal, a correction voltage according to a deviation of the pull-in voltage ΔVd for each of a plurality of display areas in horizontal direction.